I do use the mod, yes, @LuckySimmer10.
If a ghost is not available to select for summon an urn, it means the ghost is no longer in the game at all. It was probably culled.
As far as the difference between summoning and binding. Summoning gets you an urn that is already bound to that particular ghost. Binding binds an urn that already exists to a specific ghost. You never need to do both.
Think of it this way. If there is a dead sim for which you need the urn, you "summon". If you have a urn that needs a ghost, you use "bind". Binding is useful if you have a graveyard, for example, because over time the ghosts associated with the graves are culled. Instead of having to replace gravestones with new ones that do have ghosts, it lets you bind new ghosts to those that are already in place.
I suggest you read the mod's description page as it explains all of that as well as the other features pretty well.
